1. Field of the Invention
Twin-shafted grinders are commonly used for particle size reduction of solids in various municipal wastewater applications. Cutters separated by spacers stacked on counter-rotating shafts “grind” friable materials or “shred” woven and fibrous materials that enter the sewer system either by being flushed down the toilet or by entering through storm drains on the street. The geometry of the cutters and the shaft speeds affect the particle size and throughput produced by the machine.
2. Description of the Related Art
Since the use of non-dispersible wipes (baby wipes) and other paper products started becoming more and more prevalent by consumers, they have also been disposed of in municipal wastewater. Common cutter designs and shaft speeds have proven reasonably effective at shredding sheets of woven and fibrous materials into strips; however, these strips can weave together in the waste stream causing clogged pipes and damaging downstream equipment such as lift pumps. The related art cutter geometry and shaft speeds tend to be relatively ineffective at cutting the strips of shredded material in a second dimension to consistently produce smaller particle sizes that are less like to reweave.
As the grinder operates, the tip of a cutter tooth pulls material into the cutting chamber where the material is sheared on each side of the cutter by the adjacent cutters on the opposite shaft. There is a potential for the sheared strip to get wedged between the two cutters on the opposite shaft and pack into the void around the outside of the spacer that is between those two cutters. Twin-shafted grinders commonly operate with different shaft speeds to promote a tearing action of the material at the cutter shearing surfaces, but there is a trade-off to having the different shaft speeds. While the cutter teeth on the high-speed shaft readily clean out the material between the cutters on the low-speed shaft, the cutter teeth on the low-speed shaft are often ineffective at cleaning out the material between the cutters on the high-speed shaft, and the material that is cleaned out is inconsistent in size. In addition, accelerated wear occurs on the low-speed shaft cutter teeth relative to the wear on the high-speed shaft cutter teeth because the low speed cutters are continuously rubbing against debris that is wedged between the cutters on the high speed shaft.